In hydraulic brake systems, in response to the operation of the vehicle brake pedal, pressure is generated in two service brake circuits that are filled with brake fluid, and, thus, the service brake is hydraulically operated. For this purpose, the service brake has multiple hydraulically actuable brake cylinders arranged at the axles or wheels of the vehicle. As a result of the operation of the brake pedal, the hydraulic pressure is built up in a master brake-cylinder, and is provided to the service brake circuits directly in the case of a first known brake system or via a relay valve in the case of a second known brake system. If too little brake fluid is available when the brake pedal is not actuated, brake fluid flows out of a storage tank into the master cylinder. By contrast, during an operation of the brake pedal, the connection to the storage tank is shut off such that a service brake pressure can build up and the brake fluid does not flow back into the storage tank.
It is also known, in the case of hydraulic brake systems, to provide a brake force booster that assists the operation of the master cylinder by means of the brake pedal such that the brake pressure in the brake circuits can be built up with a relatively low operational force on the brake pedal. The brake force booster is often realized by way of a hydraulic booster circuit, which uses a hydraulic fluid or a hydraulic oil, but not brake fluid, as the pressure medium. The hydraulic booster circuit generally supplies pressure to multiple devices on the vehicle, specifically to a hydraulic steering assistance device, the pressure being built up by means of a steering assistance pump in the hydraulic booster circuit.
In the case of the first known brake system noted above, a service brake modulator, which provides an anti-lock brake function, is connected upstream of the service brake or of the brake cylinders. The service brake modulator has multiple valves and a return pump for each brake circuit, such that, through corresponding actuation of the valves, the hydraulic pressure at the individual brake cylinders can be lowered in a targeted manner below the service brake pressure modulated by means of the master cylinder, and the brake on wheels at which locking or a locking tendency is detected can be released, or the brake pressure at the brake can be at least reduced.
The known first brake system provides an anti-lock function for a hydraulic brake system by means of the inexpensive service brake modulator. A disadvantage of this brake system is that an active brake pressure build-up, for example for traction control or for electronic stabilization of the vehicle, is not possible, or is at best possible to a restricted extent with additional valves in the service brake modulator.
The second known brake system noted above is the so-called “hydraulic power brake”, which makes do without the brake force booster for the actuation of the master cylinder and without the service brake modulator with the return pumps, and permits brake force boosting, an anti-lock function and, additionally, an active brake pressure build-up in a different way. For this purpose, the service brake pressure is built up in the service brake circuits, in each case, by means of a brake fluid pump. The brake circuits are fed from a second storage tank, which is connected in unpressurized fashion to the storage tank for the supply to the master brake cylinder. By contrast, the service brake circuits are not fed with brake fluid by the master brake cylinder. A hydraulic pressure generated by means of the master brake cylinder serves merely for the actuation of relay valves in the service brake circuits, which relay valves control the pressure at the brake cylinders. For the anti-lock function, valves are connected upstream of the brake cylinders, by means of which brake fluid can be discharged to the second storage tank, and the brake can thus be released.
For an active brake pressure build-up, the relay valves are actuated not only by means of the master cylinder, but also hydraulically. Furthermore, the second known brake system also permits the actuation of a parking brake, wherein the parking brake is supplied with brake fluid, or with the necessary brake pressure for the release of the parking brake, from one of the brake circuits via a switchable valve.
The second known brake system has the disadvantage that a brake fluid pump is required for each brake circuit, and, in each case, one pressure vessel connected downstream of the brake fluid pump is required for each brake circuit. The brake fluid pumps and the pressure vessels have a disadvantageously large space requirement, and are expensive.